The present invention relates to a safety switching device for fail-safely disconnecting an electrical load, and more particularly to a safety switching device employing a new type of safety relay having a plurality of mechanically coupled contacts.
Safety switching devices in terms of the present invention are typically used for fail-safely disconnecting an automated technical system which can pose a danger to life and limb of persons during operation. In this context, “fail-safe” means that disconnecting of the dangerous system must be ensured even when a fault occurs in the safety-relevant components of the system, including the safety switching device, for example due to a component failure or damage to a cable. Safety switching devices are therefore subject to particular requirements which are defined, in particular, in relevant standards for machine safety (ISO 13849, IEC 61508, inter alia). Since these standards define different requirements depending on the degree of risk, safety switching devices in terms of the present invention are to be, in the following, primarily such devices that ensure single fault safety within the meaning of SIL3 according to IEC 61508 and/or PL e according to ISO 13849 during the safe disconnecting of an electrically operated machine.
DE 10 2005 048 601 B3 discloses a prior art safety switching device comprising a logic part which includes two microcontrollers. The two microcontrollers can communicate with each other via a data link, in order to compare their data and monitor each other. The two microcontrollers process input signals that are redundant with respect to each other and that can originate, for example, from an emergency stop button, a guard door, or a light barrier. On the output side, each microcontroller controls an electromechanical safety relay comprising a plurality of mechanically coupled and, therefore, positively driven relay contacts. In each case, one normally open contact of one relay is connected in series to a normally open contact of the other relay, and so the redundantly operating microcontroller can disconnect a power supply path to an electrical machine in two ways, i.e., in a two-channel manner. In addition, each relay has a positively driven, normally closed contact which, due to the positive driven operation, is always open when the mechanically coupled, normally open contacts are closed. The normally closed contacts of the two safety relays are also arranged in series with one another and form a current path, via which a monitoring signal is fed back to the redundant microcontrollers. The microcontrollers can check the opened contact point of the normally open contacts on the basis of the monitoring signal, in order to detect, in particular, a bonding, fusing, or the like of the normally open contacts at an early point in time, before a dangerous electrical machine is switched on.
The use of two separate electromechanical safety relays, each of which has a plurality of mechanically coupled normally closed and normally open contacts, has been common practice—established for many years—for implementing safety switching devices in terms of the present invention, as described, for example, in a publication by the authors Eberhard Kirsch, Jürgen Steinhäuser, and Friedrich Plappert titled “Sicherheitsrelais—Elementarrelais mit zwangsgeführten Kontakten; Besondere Eigenschaften und deren Nutzen” (Safety Relays—Elementary Relays Comprising Positively Driven Contacts; Particular Properties and Their Benefits). This publication is made available by ZVEI—Zentralverband Elektrotechnik—und Elektronikindustrie e. V. (Germany's Electrical Industry), 60528 Frankfurt, under the www.schaltrelais.de/download/sicherheitsrelais.pdf and, as documented, was originally made available by the cited source in 2006.
A further overview of the use of safety relays for ensuring the so-called functional safety of automated systems is provided in the publication “Von Europa in die Welt! Funktionale Sicherheit wird zum Leitgedanken” (“From Europe Out Into the World! Functional Safety Becomes a Guiding Principle”), which was published in November 2014 by ZVEI and has also been made available under www.schaltrelais.de.
In addition, safety switching devices that utilize semiconductor switching elements instead of positively driven (electromagnetic) relays for the fail-safe disconnecting of an electrical load have been available for approximately 15 years. One example of such a safety switching device is disclosed by EP 1 262 021 B1. However, safety switching devices employing electromechanical safety relays have advantages for many applications, since, by way of example, they do not require a shared reference potential with the load to be disconnected, and they will therefore also be required in the future for numerous applications.
The use of two electromechanical safety relays, each of which comprises a plurality of normally open and normally closed contacts mechanically rigidly connected to each other, is a large cost factor in prior art safety switching devices having potential-free outputs. The costs for the safety relays make up a substantial portion of the total manufacturing costs for a safety switching device.